express.Router: The express.Router class can be used to create modular mountable route handlers. A Router instance is a complete middleware and routing system; for this reason it is often referred to as a "miniapp".

module.exports = router; The example creates a router as a module, loads a middleware in it, defines some routes, and
mounts it on a path on the main app.

router need be added for module.exports(module.exports = router;), then only that router will be available.

Add below line in app.js after app.use('/users', users);

var abcd = require('./routes/abcd');

Add below line in app.js after users variable.

var abcd = require('./routes/abcd');

The app directory structure looks like the following.

Do install once again

npm install

Once done with succesfull install, use below command to start application

npm start

Then hit this url http://localhost:3000/ in browser to access the app.